1. Field of the Invention
The invention relates to a device and method for correcting a welding path of an automatic welding apparatus and, more particularly, to a device and method for correcting the welding path of an automatic welding apparatus for a welding torch to track a desired welding path precisely.
2. Background of the Invention
Generally, an automatic welding apparatus performs welding by weaving the welding torch in the up-down and the left-right directions of the welding path. When the welding torch deviates from the predetermined welding path, the automatic welding apparatus detects the welding current change and corrects the welding path letting the welding torch track the welding path.
According to the conventional method of correcting the welding path of an automatic welding apparatus, the correction of the welding path is performed at every half cycle of the weaving of the welding torch and welding is done by weaving the welding torch in the left-right direction along the welding path. An integrated value of welding current of the welding torch is computed every half cycle of weaving so that the correction direction and correction amount are decided, which is described in U.S. Pat. No. 5,066,847 assigned to Fanuc Ltd., Japan.
FIG. 1A through FIG. 1C are views for describing the conventional method described above of correcting the welding path in the left-right direction.
FIG. 1A illustrates the welding path between a welding position P.sub.1 and a welding position P.sub.2 during a half cycle of weaving from time t.sub.1 to time t.sub.2.
FIG. 1B shows a comparison of two integrated values of the welding current obtained in between time t.sub.1 and t.sub.2, one between t.sub.0 and t.sub.1 and the other between t.sub.0 and t.sub.2, where time t.sub.0 =(t.sub.1 +t.sub.2)/2. The integrated value of the welding current S.sub.1 between t.sub.0 and t.sub.1 and the integrated value of the welding current S.sub.2 between t.sub.0 and t.sub.2 are compared, and the correction is made in the direction which increases of the smaller of the two, S.sub.1 and S.sub.2. In other words, when S.sub.1 &lt;S.sub.2, correction is made the correction is made in the direction S.sub.1 and when S.sub.1 &gt;S.sub.2, in the other direction. Here, the correction amount T1 is, T1=[(S.sub.2 /S.sub.1)-1].times..DELTA..sub.1 where T1 is correction amount and .DELTA..sub.1 represents a coefficient.
FIG. 1C shows the method of correcting the welding path in the left-right direction of the conventional method, in which the correction direction of welding path is indicated by the arrow P with respect to the weaving surface.
FIGS. 2A and 2B show the method of correcting the welding path in the up-down direction with respect to the weaving surface of the conventional method. In FIG. 2A, the integrated value S.sub.3 during the half cycle of weaving between time t.sub.1 and time t.sub.2 is preset, and then the integrated value of actual welding current S.sub.4 =S.sub.3 +S.sub.3 ' is determined. Then, the integrated value S.sub.3 is compared with the integrated value S.sub.4 so as to correct the welding path in a manner that when S.sub.3 &lt;S.sub.4, the direction of the position correction is made downward, the torch approaching the workpiece, and when S.sub.3 &gt;S.sub.4, the direction of the position correction is made upward, the torch moving away from the workpiece.
Here, the position correction amount T2=[(S.sub.4 /S.sub.3)-1].times..DELTA..sub.2, where .DELTA..sub.2 represents a coefficient. The direction of the position correction in this case is the up-down direction as indicated by the arrow q with respect to the weaving surface (FIG. 2B).
FIG. 3A shows the comparison of welding current in a flux cored arc welding apparatus with varying welding conditions. As shown, the welding current change is irregular because of the noise effect.
FIG. 3B shows the experimental model in the flux cored arc welding process.
Here, when the value of the welding current applied to the automatic welding apparatus is small, the change of welding current with respect to the change of the tip-to-workpiece distance is small, and hence the sensitivity of the welding current is also small. For example, as shown, if flux cored wire is used instead of solid wire, then the welding current applied becomes small and the slope described in FIG. 3B becomes small thereby making the sensitivity of the welding current small and, as a result, the welding current sensing becomes difficult.
Also, as the welding in the vertical direction employs lower current than in the horizontal one, the sensitivity of the welding current is even lower in vertical direction.
According to the conventional method of correcting the welding path described above, the method comprises the steps of: computing the reference integrated value of the welding current of the welding torch for every half cycle of weaving; determining the direction of correction by comparing the reference integrated value with an actual integrated value of half cycle of weaving; determining a correction amount in a direction which will vary a distance between the weaving surface and the workpiece depending upon a difference in the actual integrated value with respect to the reference integrated value; and correcting the welding path by multiplying the correction amount by a predetermined coefficient.
However, the conventional method of correcting the welding path has the following problems. Specifically, in detecting the welding current of the automatic welding apparatus, noise or drastic changes of current may occur and the change of the actual current during welding is irregular. If the tip-to-workpiece distance increases, the welding current decreases and especially, if the welding torch is slowly biased, it is difficult to discriminate between the welding current and noise, and hence it is difficult to obtain a reliable value of the welding current.
Accordingly, it is difficult to obtain an adequate correction because the increase or decrease of integrated value of the welding current occurs substantially at every half cycle of weaving, and hence it is difficult to set the reference integrated value and the coefficient for the correction amount.